Solid-state image display devices utilizing light-emissive pixels are well known and widely used. For example, OLED devices are used in flat-panel displays, in both passive- and active-matrix configurations, and in both top-emitter and bottom-emitter designs. Control circuits for OLED displays are also well known in the art and include both voltage- and current-controlled schemes.
Conventional passive-matrix OLED displays employ drivers to conduct current through an OLED element over a fixed period (also known as a frame or frame period) during which the OLED light-emitting element emits light at a specific luminance. Successive rows or columns of OLED elements are energized and the entire OLED display is refreshed at a rate sufficient to avoid the appearance of flicker. For example, WO 2003/034389 entitled, “System and Method for Providing Pulse Amplitude Modulation for OLED Display Drivers,” published Apr. 24, 2003, describes a pulse width modulation driver for an organic light emitting diode display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light emitting diode in a video display. The voltage driver may receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics.
In contrast, active-matrix circuits employ a two-dimensional array of individual circuits for each light-emitting element in a display. The active-matrix circuit provides a control mechanism for storing a value (typically as a charge on a capacitor) that is then employed to control a drive circuit to provide current through the light-emitting element (also known as a pixel or sub-pixel). As used herein, each light-emitting element is considered to be a pixel, regardless of color or grouping with other light-emitting elements. For example, referring to FIG. 12, an active-matrix pixel circuit for driving an LED 10 includes a control transistor 12 responsive to control signals such as a select signal 14 and data signal 16. Upon activation of select signal 14, the control transistor 12 is turned on and data signal 16 provides a charge to a storage capacitor 20. The control transistor 12 is subsequently turned off by deactivation of select signal 14. The charge stored on the storage capacitor 20 turns on driving transistor 22 to provide current to LED 10 at a level commensurate with the charge stored on capacitor 20. Referring to FIG. 13a, a pixel might emit light at a luminance level L1 during a first frame period T1 and at a second luminance level L2 during a second frame period T2. The changes in luminance are perceived by an observer as changes in an image, for example, motion in a scene.
In a conventional, prior-art flat-panel display, a display signal is typically refreshed periodically at a rate high enough to provide the appearance of smooth motion in sequential frames of a video stream. Refresh rates are typically 30, 60, 70, 75, 80, 90, or 100 frames per second for monitors, 50 or 60 frames per second for televisions. Hence, in a conventional flat-panel display, the charge in the charge storage capacitor 20 is updated at the selected refresh rate appropriate to the application.
The luminance value at each pixel is typically refreshed at a refresh rate (for example 30 Hz or 60 Hz) defining a frame period. The frame period is chosen to be sufficiently short so that the illusion of motion is provided when the luminance values of the pixels change. As is known, such active-matrix circuits can cause motion blur in observers, because the image is static during a frame period while an observer's eye may track across the display, exposing the image to different portions of the retina. This blur can be reduced by reducing the period of the refresh, that is refreshing at a higher frequency. However, such a solution is problematic, in that higher frequency signals are employed, raising the cost of drivers and exacerbating transmission line effects in the control lines used to store charge at each pixel location. Alternatively, the time during each frame for which the pixel is emitting light may be reduced, for example, by emitting brighter light during only a portion of the frame time. If the frame period is sufficiently short, no flicker will be perceived. Referring to FIG. 13b, during a first frame period T1, a pixel may be controlled to emit twice the light 2L1 during one half of the period T1 and similarly emit light at twice the luminance level 2L2 during one half of the period T2. In a related solution, portions of a display may display a black bar that scrolls across the display. However, these solutions also require higher-frequency controls that raise costs and are problematic for larger displays with longer control lines.
Known pulse-width modulation techniques may be employed to control a display pixel as illustrated in FIG. 13b. Moreover, because one source of non-uniformity in an OLED display results from variability in the threshold switching characteristics of thin-film drive transistors employed in active-matrix designs, one approach to improving uniformity in an active-matrix OLED display is to employ pulse-width modulation techniques in contrast to charge-deposition control techniques. These pulse-width modulation techniques operate by driving the OLED at a maximum current and brightness for a specific first amount of time and then turning the OLED off for a second amount of time within the same frame time. If the sum of the first and second amounts of time is sufficiently small, the flicker resulting from turning the OLED on and off periodically will not be perceptible to a viewer. The brightness of the OLED element is controlled by varying the ratio of amount of time that the OLED is turned on in comparison to the amount of time that the OLED is turned off.
A variety of methods for controlling an OLED display using pulse-width modulation are known. For example, U.S. Pat. No. 6,809,710 entitled, “Gray scale pixel driver for electronic display and method of operation therefore” granted Oct. 26, 2004, discloses a circuit for driving an OLED in a graphics display. The circuit employs a current source connected to a terminal of the OLED operating in a switched mode. The current source is responsive to a combination of a selectively set cyclical voltage signal and a cyclical variable amplitude voltage signal. The current source, when switched on, is designed and optimized to supply the OLED with the amount of current necessary for the OLED to achieve maximum luminance. When switched off, the current source blocks the supply of current to the OLED, providing a uniform black level for an OLED display. The apparent luminance of the OLED is controlled by modulating the pulse width of the current supplied to the OLED, thus varying the length of time during which current is supplied to the OLED.
By using a switched mode of operation at the current source, the circuit is able to employ a larger range of voltages to control the luminance values in a current-driven OLED display. However, use of current-driven circuits is complex and requires a large amount of space for each pixel in a display device.
There are also methods known for providing both a pulse width control and a variable charge deposition control in a single circuit. U.S. Pat. No. 6,670,773 entitled, “Drive circuit for active matrix light emitting device,” suggests a transistor in parallel with an OLED element. The described technique, however, diverts driving current from an OLED, thereby, decreasing the operating efficiency of the circuit. Other designs employ circuit elements in series with the OLED element for controlling or measuring the performance of the OLED element. For example, WO 2004/036536 entitled, “Active Matrix Organic Electroluminescent Display Device” published Apr. 29, 2004, illustrates a circuit having additional elements in series with an OLED element. However, when placed in series with an OLED element, transistors will increase the overall voltage necessary to drive the OLED element or may otherwise increase the overall power used by the OLED element or decrease the range of currents available to the OLED element.
In U.S. Pat. No. 7,088,051, by Cok, issued Aug. 8, 2006, a pulse-width modulation scheme with a variable control is disclosed and is hereby incorporated in its entirety by reference. This disclosure describes a means for controlling the luminance of a pixel during a frame time; however, external control is required, thereby increasing costs and reducing aperture ratio of the device.
There is a need, therefore, for an improved control circuit for active-matrix OLED devices having a simplified and flexible design.